Minute Inhabitants of Aquaria 



PERRY BRUCE CLARK, San Francisco Aquarium Society 



The rotifers, or wheel animalcules, are 

 very interesting animals, very transpar- 

 ent, and their structure is easily seen 

 with a microscope. They are small, 

 measuring one-thirtieth of an inch, but 

 are highly organized. Circular discs at 

 the anterior or mouth end, from which 

 project a row of cilia which are in con- 

 stant motion, greatly resemble revolving 

 wheels. These cilia or hairs sweep the 

 food into the mouth, and also serve as 

 propellers for swimming. It may also 

 move after the manner of the so-called 

 measuring worm by loping, that is, at- 

 taching the head and then moving the tail 

 up to it. The rotifers multiply by means 

 of eggs. It has been stated that these 

 eggs are very resistant, and may under 

 favorable conditions develop after hav- 

 ing been in the dry state for a number 

 of years. At times rotifers may be col- 

 lected in quantity and form a valuable 

 food for fisli just out of the egg. 



Nearly all the live foods used for 

 young fishes belong to the class Crust, 

 acea, which contains some very small ani- 

 mals, as well as some large ones. As the 

 name implies, they are shell-bearing ani- 

 mals. The body is segmented, though in 

 some of the smaller species it is very in- 

 distinct. In this class belong the lobster, 

 crab, shrimp, crayfish, and smaller form 

 such as Gainarus, Daphnia, Cyclops and 

 Cyrpris. Of these the best for fish food 

 is Daphnia, the body of which is flat- 

 tened, indistinctly segmented, and en- 

 closed in a bivalve carapace or shell. The 

 prominent antennae give it a bizarre ap- 

 pearance as it jerks its way through the 

 water; they look like abnormally long 

 arms waved above its head. It multi- 



plies rapidly in a very curious manner. 

 During the summer the female produces 

 eggs, which, without fertilization by the 

 male, mature in the brood pouch into 

 perfect animals in about four days. A 





Daphnia pulex (greatly enlarged) 

 Original Drawing by Frank J. Myers 



few days after leaving the mother they, 

 too, reproduce in like manner. In the 

 fall of the year the much smaller males 

 appear, and then sexual reproduction 

 takes place. The female now lays winter 

 eggs, having a thick covering, which pro- 

 tects them through the cold months until 

 they hatch. Daphnia live upon decaying 



